2 Answers
2

You can measure the gravitational acceleration with a pendulum and a stopwatch, which is how Newton did it. However these days it's measured to great precision using accelerometers aboard satellites. In particular the GRACE and GOCE satellites have measured the gravitational field of the Earth to exquisite precision.

did Newton have a stopwatch? Galileo had to use heartbeats, but I think Newton might have had a pendulum clock.
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Ron MaimonNov 2 '12 at 17:07

Actually I'm not sure that Newton measured $g$, or at least not to any special accuracy. He did do experiments with pendulums (penduli?) but I think it was to show that different materials gave the same period.
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John RennieNov 2 '12 at 18:02

I suspect you are right, but then I am not sure when "g" was measured, or by whom. Galileo probably had a crappy measurement like "8.9 $\pm$ .6 (Galileo armspans) per (Galileo heartbeat)^2 ". Maybe it was Huygens. He was designing clocks, I think.
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Ron MaimonNov 2 '12 at 18:07

Galileo measured the time of falling from the leaning tower of pisa. Don't know what precision he had on the value of little g.
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Jerry SchirmerAug 3 '13 at 21:23